Organic field effect transistors (OFETs) are used in display devices and logic capable circuits. For many display applications a high on/off ratio is required so that sufficient contrast and a reliable dynamic operation is obtained. Improvements in the on/off ratio have been achieved in the past by patterning of the organic semiconductor (OSC) layer to reduce parasitic leakage currents, as disclosed for example in U.S. Pat. No. 5,854,139. The semiconductor layer may be patterned by conventional lithography using an etching step to remove the layer from unwanted areas. Such techniques are well established in inorganic electronics. However, organic semiconductors (OSC) are often not easily processed by etching. For example the OSC layer may be deposited by inkjet on active areas only. U.S. Pat. No. 5,854,139 discusses patterning an oligothiophene OSC by illuminating the OSC layer through an optical mask.
WO 2006/048092 A1 suggests a process for patterning the OSC layer by applying an oxidizing agent to the OSC layer outside the channel areas, and reports that this leads to an improved on/off ratio.
Another method reported for patterning the OSC layer suggested in prior art is based on varying the surface energy (wettability) of the surface around a transistor device. This concept of directing or confining a deposited drop via a surface energy gradient is a well known principle and is reported as being especially suitable for inkjet printing processes, as described for example in Appl. Phys. Lett. 79, 3536 (2001).
However, the methods described in prior art do still leave room for further improvements.
It is therefore an aim of the present invention to provide methods for manufacturing improved organic electronic (OE) devices, in particular bottom gate (BG) OFETs. In particular, it is an aim to improve the on/off ratio in an OFET and maintain the mobility of the layer in the channel area whilst reducing the mobility in other areas. The method should not have the drawbacks of prior art methods and allow time-, cost- and material-effective production of electronic devices at large scale. Other aims of the present invention are immediately evident to the expert from the following